RATIONALE: Sufficient levels of vitamin D seem to be essential for proper immune function, and low levels might be associated to disease activity in Rheumatoid Arthritis (RA). Most studies investigate only 25OHD and not the physiologically active vitamin D metabolite, 1,25(OH)2 D.

OBJECTIVE: To investigate associations between serum level of vitamin D metabolites and disease activity parameters in 160 inflammatory active and treatment naïve early RA patients. Serum level of vitamin D metabolites (25OHD2 , 25OHD3 and 1,25(OH)2 D) was measured by isotope dilution mass spectrometry and radio-immunoassays at baseline. Disease characteristics were gender, number of tender joints, number of swollen joints, DAS28-CRP, HAQ, VAS-scores, CRP, erosive status (Total Sharp Score; TSS), ACPA and IgM-RF-status. Associations were evaluated using Spearman's and Wilcoxon rank-sum tests. The study was registered in clinical trials; trial registration number: NCT00209859.

FINDINGS: Statistically significant inverse associations were found between the active metabolite 1,25(OH)2 D and DAS28-CRP (P = 0.004, rho = -0.23), HAQ (P = 0.005, rho = -0.22), CRP (P = 0.001, rho = -0.25), VASpatient-pain (P = 0.008, rho = -0.21), and a positive association was found to ACPA-status (P = 0.04).

CONCLUSION: The vitamin D metabolite 1,25(OH)2 D was inversely associated with disease activity and positively associated with ACPA in treatment naïve and inflammatory active early RA. The results indicate that in RA, both the degree of inflammatory activity, and the diagnostic sensitivity and specificity might affect-or might be affected by the level of vitamin 1,25(OH)2 D.